As is known, integrated circuits (ICs) are devices that include a multitude of transistors and other active circuits arranged and configured on a semiconductor substrate, such as silicon or gallium arsenide, to perform certain functions. During operation of an IC, the active circuits in the IC generate heat. Packages that hold the ICs typically include elements, such as heat sinks, intended to transfer much of the generated heat away from the active circuits. Failure to transfer the heat can result in undesired changes in IC performance or, worse yet, IC failure.
A cross section of a typical IC package 100 is depicted in FIG. 1. The IC package 100 includes a semiconductor substrate 101 or die, an adhesive layer 103, an internal printed circuit board (PCB) substrate 105, an internal epoxy molding material 107, an internal heat sink 109, and an external epoxy molding material 111. The semiconductor substrate 101 includes multiple active circuits 113 (e.g., transistors) that are connected to conductive traces 115 on the internal PCB substrate 105 via wire bonds 117 or other electrically conductive paths. For example, in flip chip technology, the semiconductor substrate 101 is flipped over (hence the name “flip chip”) such that the connecting terminals of the active circuits 113 can be directly soldered or otherwise connected (e.g., through the use of conductive epoxy) to the conductive traces 115 or pads of the internal PCB substrate 105, thereby eliminating the need for the wire bonds 117 and the adhesive layer 103 connecting the semiconductor substrate 101 to the internal PCB substrate 105. The IC package 100 typically further includes solder balls 119 to allow the IC package 100 to be wave soldered or otherwise electrically connected to conductive traces or pads 121 of an external printed circuit board 123. The IC package 100 depicted in FIG. 1 is typically referred to as a plastic ball grid array (PBGA) package.
The adhesive layer 103 is typically a thin layer of conductive epoxy used to position the semiconductor substrate 101 in a fixed relationship to the PCB substrate 105 using known die attachment techniques. The internal epoxy molding material 107 is used to keep the wire bonds 117 from coming into contact with the internal heat sink 109 and to match the coefficient of thermal expansion (CTE) of the semiconductor substrate 101 to the CTE of the internal heat sink 109. As mentioned above, the internal heat sink 109 is included to transfer or conduct heat generated by the active circuits 113 of the semiconductor substrate 101 away from the semiconductor substrate 101. The internal epoxy molding material 107 is typically an epoxy resin with relatively poor thermal conduction properties as compared to the internal heat sink 109 (which is typically copper or aluminum) or the semiconductor substrate 101 (which is typically silicon or gallium arsenide). Typically, the thermal conductivity of the internal epoxy molding material 107 is four hundred (400) to four hundred fifty (450) times worse than the thermal conductivity of the internal heat sink 109 and one hundred fifty (150) to one hundred sixty (160) times worse than the thermal conductivity of the semiconductor substrate 101. Thus, the internal epoxy molding material 107 serves as a substantial barrier to the rapid and efficient transfer of heat away from the semiconductor substrate 101.
One prior art technique for reducing the thickness of the internal epoxy molding material 107 to improve heat transfer from the semiconductor substrate 101 to the internal heat sink 109 is depicted in cross section in the IC package 200 of FIG. 2. As illustrated in FIG. 2, the configuration of the internal heat sink 201 has been changed to include a downward extrusion in a center portion of the heat sink 201 to reduce the thickness of the internal epoxy molding material 107 and, accordingly, the distance the heat generated by the semiconductor substrate 101 must travel to reach the heat sink 201. Although such a change in the configuration of the heat sink 201 improves heat transfer, the extruded heat sink 201 is costly and there still exists a poor thermal conduction layer between the semiconductor substrate 101 and the heat sink 201.
Another alternative is to directly connect the extruded part of the heat sink 201 to the semiconductor substrate 101 using a very thin, thermally conductive adhesive (e.g., conductive epoxy, such as is used for die attachment). Although such a direct connection would provide optimal heat transfer, mismatches in the CTEs of the heat sink 201 and the semiconductor substrate 101 would result in poor reliability of the IC package 200 over temperature (e.g., the semiconductor substrate 101 would likely crack over time due to the mismatches in CTE). Also, the metal heat sink 201 may short circuit or otherwise negatively impact the performance of the circuits 113 disposed on the semiconductor substrate 101 if the heat sink 201 is directly connected to the substrate 101.
Vertical stacking of active semiconductor substrates is also known for reducing the printed circuit board area for a particular amount of functionality. An exemplary IC 300 that utilizes vertical stacking is depicted in cross section in FIG. 3. As shown in FIG. 3, two semiconductor substrates 301, 303 are stacked vertically. Each semiconductor substrate 301, 303 includes respective active or heat-generating circuits 305, 307. The active circuits 305, 307 are connected to respective traces or pads 309, 311 on a PCB substrate 313 via wire bonds 315, 317 or equivalent conductive paths. The two substrates 301, 303 are connected together via a thin, electrically non-conductive adhesive layer 319, such as epoxy, and the lower substrate 303 is connected to the PCB substrate 313 via a thin adhesive layer 321 that may be electrically conductive (e.g., conductive epoxy) or electrically non-conductive (e.g., epoxy). The adhesive layer 319 connecting the two substrates 301, 303 together can be very thin because, in most cases, the CTEs of the two substrates 301, 303 are substantially identical (i.e., the two substrates 301, 303 are typically the same (e.g., both silicon or both gallium arsenide)). Although not depicted in FIG. 3, the IC 300 is typically encased by an internal epoxy molding material, an internal heat sink, and an external epoxy molding material as discussed above with respect to FIGS. 1 and 2. Thus, although known for providing increased functionality in a fixed PCB area, vertical stacking of multiple heat-generating semiconductor substrates 301, 303 provides no improvement in transferring the heat generated by the semiconductor substrates 301, 303 away from the substrates 301, 303.
Therefore, a need exists for an integrated circuit package and corresponding method of fabrication that improve the transfer of heat generated by an integrated circuit away from the integrated circuit, without sacrificing package reliability.